1. Field of the Invention
The present invention relates to a heat treatment furnace for fiber and, more particularly, to a heat treatment furnace (an oxidizing heat treatment furnace, an oxidizing furnace, or the like) for producing an oxidized fiber needed to produce a carbon fiber. The invention also relates to a yarn guide roller for use in the heat treatment furnace.
2. Description of the Prior Art
A horizontal-type heat treatment furnace described in Japanese examined patent application publication No. Hei 3-4832 is known as a heat treatment furnace used for an oxidizing heat treatment of a precursor fiber bundle in order to obtain an oxidized fiber bundle. The horizontal-type heat treatment furnace has a furnace body, a plurality of heat treatment chambers provided in the furnace body, a hot gas blow opening and a hot gas suction opening that are formed in each heat treatment chamber, a hot gas circulation duct to which the plurality of hot gas blow openings and the hot gas suction openings are commonly connected, a heater provided in the hot gas circulation duct, and a hot gas-circulating fan disposed downstream from the heater. That is, this conventional heat treatment furnace is a multi heat treatment chambers/common hot gas circulation duct type heat treatment furnace that circulates a hot gas and maintains a predetermined temperature of the hot gas by using the hot gas-circulating fan and the hot gas-heating heater provided in the hot gas circulation duct connected to the hot gas blow openings and the hot gas suction openings of the heat treatment chambers.
A precursor fiber bundle (yarn) used to produce an oxidized fiber bundle for production of a carbon fiber bundle, for example, a fiber bundle (yarn) formed of a great number of polyacrylonitrile (PAN)-based continuous filaments, moves along a zigzag path, guided by a plurality of yarn guide rollers provided outside the heat treatment furnace, so that the fiber bundle sequentially passes through the heat treatment chambers. The fiber bundle receives oxidizing treatment during the passage through the heat treatment chambers. However, the heat treatment furnace has the following problems.
The oxidation of precursor fiber bundles gradually progresses. If a yarn is treated at a high temperature in an early stage of heat treatment, the yarn is likely to fire because the oxidation has not fully progressed in that stage. Therefore, it is necessary to maintain a low heat-treating temperature until the oxidation of yarn progresses to a certain extent. However, if a low temperature setting continues in a later stage of the heat treatment, a long heat treatment time is required. To secure a long heat treatment time, there arises a need to increase the furnace length or the number of passages through the furnace (that is, the number of paths in the furnace along which yarn is moved). As a result, the scale of the furnace becomes great, or the equipment cost increases, or economical production of carbon fiber bundles, which are produced by carbonizing oxidized fiber bundles, becomes difficult.
In view of these problems, the aforementioned conventional heat treatment furnace will be examined. In order to avoid firing of a precursor yarn in a heat treatment chamber into which a yarn is first introduced, a low temperature setting is needed in the first heat treatment chamber of a heat treatment furnace. However, since the conventional heat treatment furnace is a multi heat treatment chambers/common hot gas circulation duct type heat treatment furnace, the temperatures in the heat treatment chambers succeeding to the first heat treatment chamber inevitably become equal to the temperature in the first heat treatment chamber. Therefore, the heat treatment time for precursor yarns (the length of time during which a yarn is treated in the heat treatment chambers) inevitably becomes long in the conventional heat treatment furnace, thereby causing problems of increased length and scale of the heat treatment furnace and, therefore, increased equipment and production costs.
Furthermore, in order to vary the heat treatment temperature in accordance with the progress of oxidation of a precursor yarn in the conventional heat treatment furnace, it is necessary to use a plurality of heat treatment furnaces that differ in heat treatment temperature. However, this requirement increases the equipment installation space, the equipment cost and, therefore, the production cost of carbon fibers.
A known yarn guide roller as described above is described in Japanese examined patent application publication Sho 59-28662. This yarn guide roller has a guide groove that is formed on a peripheral surface of the roller for guiding a yarn. The groove forms a circular sectional shape of the yarn that is introduced into the heat treatment chambers. However, as the denier or the number of filaments of a yarn guided by the groove, the maximum yarn thickness increases and, therefore, yarn heat accumulation increases, so that breakage of a filament constituting the yarn becomes more likely due to the heat accumulation.
In order to avoid such an increase in the likelihood of filament breakage, it is necessary to perform oxidizing treatment at a lower temperature. Therefore, if the aforementioned yarn guide roller is used, it takes an inconveniently long time to produce a sufficiently oxidized fiber.
Furthermore, since the groove of the yarn guide roller shapes the sectional shape of a yarn into a circular shape, diffusion of oxygen, which is required for the yarn oxidation, into an interior of the yarn (filaments present inside the yarn) becomes less easy to occur. As a result, the degree of oxidation progress considerably differs between an interior portion (filaments present inside) of the yarn and a surface portion (filaments adjacent to the yarn surface) of the yarn. Such a oxidation progress difference in interior and surface portions of the yarn can become a cause for fuzzing or a damage of a filament in a later-performed carbonizing process. The conventional yarn guide roller has problems as described above.